A miniature peristaltic pump includes a cartridge received in motor frame, a rotor received in the cartridge, a sleeve mounted on the rotor and a length of compressible tubing having opposite ends, wrapped around the sleeve. The cartridge is formed by two members, the "housing" and the "cover". The rotor is a one-piece rigid member including an eccentric portion which radially squeezes the resilient tubing. The tubing is in a single loop around the rotor sleeve inside the circular pumping cartridge. The cyclic squeezing of the tubing by the rotor and the associated sleeve forces fluid from an inlet port to an outlet port of the pump.
Ideally, a disposable pump cartridge should be assembled and ready for insertion by connecting external inlet and outlet ports to tubing running from the source of medication to the patient. However, the structure of a closed continuous pump cartridge and the eccentricity of the rotor operate to pinch the resilient tubing inside the chamber. Pinching of the tubing can result in a permanent occlusion in the tubing. Further, pinching of the tubing also prevents gravity flushing of air bubbles from the system.
U.S. Pat. No. 4,559,040 focused on an improved cartridge housing wherein a chord segment was removed from the circumference of the pump cartridge housing to prevent the resilient tubing from being pinched at a set position of the rotor high point during storage. With the chord segment of the cartridge housing removed during storage, the tubing remained an open and resilient channel. When the cartridge was prepared for usage, the tubing was readily loaded with medicant by gravity flow because the fluid passage through the tubing in the cartridge is unoccluded. The chord segment was then mounted in place, and the cartridge was inserted into the a motorized drive mechanism (motor frame), wherein a motor shaft of the motor received the rotor of the cartridge to drive the rotor for delivery of medicant to a patient. However, the implementation of the above-described cartridge into a pump motor frame resulted in a modification of the proposed design. The chord segment of the pump chamber removed from the cartridge was integrated into the pump motor frame, eliminating the need for a separate pump chamber segment. However, insertion of a cartridge lacking a portion of its pump chamber into a pump motor frame sometimes resulted in the pinching of tubing between the pump frame and the cartridge at the frame/cartridge interface, producing the occlusions sought to be avoided by the solution described in the above-noted patent.
Additional improvements to the disposable cartridge used in the pump motor frame of a small peristaltic pump used in medical applications, are described in pending U.S. application Ser. No. 996,326, entitled "Improved Cartridge for Drug Infusion Pump," filed Dec. 23, 1992 and assigned to the assignee of the present invention, and to the extend that the prior application is helpful to an understanding of the present invention, such application in its entirety is incorporated by reference herein.
The solutions of the aforementioned patent and patent application focused on improvements in the cartridge housing. However, the improvements in cartridge housing structure did not completely eliminate undesirable variations in fluid flow in the cartridge.
The rotor is a major drive component of the cartridge, disposed within a flexible sleeve which is in constant engagement with the tubing, the rotor/sleeve combination cyclicly squeezing the tubing between the peripheral wall of the cartridge and the rotor to deliver fluid through the pump to the patient.
The rotor plays a key role in the rotary peristaltic infusion cartridge. The rotor/sleeve combination provides the direct interface between the output shaft of the motor drive and compression of the tubing. Within this interface there is no means of compensation for the influence of the outer limits of the design tolerances, which could cause either over-compression (high torque) or under-compression (dynamic leak back) of the tubing. High torque decreases battery life, and dynamic leak back could result in under-infusion.
However, the present rotor is a relatively rigid one-piece plastic member carrying a fixed eccentric member at its outer surface. Moreover, the cartridge housing is a relatively inflexible plastic body having an internal cylindrical cavity of fixed proportions which receives the rotor/sleeve combination. Thus the tubing wrapped around the rotor/sleeve combination is trapped between two substantially rigid members, the cartridge housing and the rotor/sleeve combination, to be cyclically squeezed by said members at substantially high levels of force under relatively inflexible conditions. Relatively slight variations or defects in cartridge housing structure or rotor structure would substantially increase or decrease force levels experienced by the tubing during the pumping cycle, to produce undesirable variations in fluid flow rates, to produce occlusions in the tubing, and to decrease battery life of the pump or to produce undercompression failures, which result in underdelivery of drug to the patient.
The cartridge of the present system would produce improved performance if its inflexible structure could be modified to minimize the effect of variations in structural tolerances, thus to increase compression levels to limit undercompression failures and also to reduce excessive forces exerted on the tubing during the pumping cycle. Because the cartridge housing is a rigid structure having fixed dimensions to maintain a consistent cartridge/motor frame interface, flexibility is not a desirable feature for incorporation into the cartridge housing. Indeed any flexibility built in the cartridge is provided to facilitate the cartridge/motor frame interface, and not to accommodate the rotor chamber within the cartridge housing.
However, it is possible to modify the rotor structure to incorporate a degree of flexibility therein to better enable the cartridge to overcome undesirable manufacturing variations which produce tubing occlusions and undesirable variations in flow rates, whether due to underdesirably low and undesirably high compression forces.